The effectiveness of a reverse osmosis water purification device can be monitored by a system which compares the conductivity of raw water supplied to the reverse osmosis device to the conductivity of treated water which has been passed through the reverse osmosis device. Such monitoring systems typically require the installation of two pairs of electronic probes, one pair is located in the flow, and spaced apart in the direction of flow of raw water to the reverse osmosis device and a second pair is located in the flow, and spaced apart in the direction of flow of processed or product water which has passed through the reverse osmosis device. The two pairs of electrodes are connected to an electronic circuit which is designed to compare the conductivities of the water flowing between the two pairs of electrodes and to provide an electrical output signal or signals to energize indicator lights or audible alarms which indicate the current operating condition of the reverse osmosis device. Further, it is necessary to provide a source of electrical power for the electronic circuit. As assembled in the past, such monitoring systems provided the two pairs of electrodes, the electronic circuit, and the source of electrical power as separate components, each of which was individually installed for use in monitoring a reverse osmosis water purification device.